1. Field of the Invention
The present invention relates generally to the fields of neurobiology and biochemistry. More particularly, it concerns methods and compositions for therapeutic intervention against Parkinson's disease. In particular, methods of making and sequestering dopamine are disclosed.
2. Description of Related Art
Parkinson's disease (PD) is an age-related disorder characterized by a loss of dopamine neurons in the substantia nigra of the midbrain. These neurons have the basal ganglia as their major target organ. The symptoms of PD include tremor, rigidity and ataxia. The disease is progressive, but can be treated by replacement of dopamine through the administration of pharmacological doses of the precursor for dopamine, L-3,4-dihydroxyphenylalanine (levodopa; L-DOPA; Marsden, 1986; Vinken et al., 1986). However, with chronic use of this pharmacotherapy, the patients often develop a fluctuating response to L-DOPA. There are many suggested mechanisms for the development of the fluctuation, but one simple explanation is that the patients reach a threshold of cell and terminal loss, wherein the remaining cells cannot synthesize and store sufficient dopamine from the precursor.
Typically PD patients are routinely treated with a combination of L-DOPA and a DOPA decarboxylase inhibitor such as carbidopa or benserazide. Unfortunately, after an initial period of satisfactory, smooth and stable clinical benefit from L-DOPA therapy, lasting on the average 2–5 years, the condition of many patients deteriorates and they develop complex dose-related, as well as unpredictable, response fluctuations. The causes of the response fluctuations are probably multiple and complex, but pharmacokinetic problems likely play some role. There is a correlation between the clinical fluctuations and the oscillations of L-DOPA plasma levels. However, many of the problems are a result of the unfavorable pharmacodynamics response to L-DOPA, i.e., a short half-life in vivo due to various central factors.
Another treatment route is through intracerebral grafting. PD is the first disease of the brain for which therapeutic intracerebral grafting has been used in humans. Preclinical and clinical data indicate that transplanted cells (the graft) used in cell transplantation protocols for these types of neurodegenerative diseases survive and integrate with the host tissue, and provides functional recovery (Wictorin et al., 1990).
Several attempts have been made to provide the neurotransmitter dopamine to cells of the basal ganglia of Parkinson's patients by transplantation of fetal brain cells from the substantia nigra, an area of the brain rich in dopamine-containing cell bodies and also the area of the brain most affected in PD. Fetal dopaminergic neurons have been shown to be effective in reversing the behavioral deficits in rat models of PD induced by selective dopaminergic neurotoxins (Bjorklund et al., 1986; Dunnett et al., 1983). The major effect of fetal transplantation in PD has been in enhancing patients' response to L-DOPA, rather than alleviating the need for the drug. This effect is thought to be due to added capacity to decarboxylate L-DOPA and storage of the formed dopamine (Lindvall et al, 1994; Sawle et al, 1992).
Non-fetal cell transplants also have been used in an attempt to combat PD, e.g., the use of chromaffin cells. A major advantage of this type of transplantation protocol is that the graft source is not a fetal source and thus, circumvents the ethical and logistical problems associated with acquiring fetal tissue. Using the chromaffin cell protocol, normalization of behavior has been observed. However, the functional recovery of this behavior is temporary and the animals revert to their pre-transplantation status (Bjorklund and Stenevi, 1985; Lindvall et al., 1987). The inability of this type of treatment protocol to maintain normal behavioral activity in animals in the PD model renders clinical application of this protocol as well as other treatment therapies ineffective.
Finally, it is known that the chemical deficit that results in PD is the inability to supply dopamine. The rate limiting enzyme in the production of dopamine, tyrosine hydroxylase, has been cloned and the anatomical localization of the affected region has been identified as the basal ganglia. Neverthless, in animal models, it has been shown that increasing the activity of tyrosine hydroxylase in Parkinsonian tissue does not result in the long-term amelioration of the symptoms of PD.
Clearly, there is a need for methods and compositions that will correct the chemical deficit in PD in a reliable manner that will lead to the treatment of the disease.